1. Field of the Invention
The present invention relates to the protection of the wall of a light water reactor fuel pin from internal forces of the pin. More particularly, the invention relates to mechanical insulation of the fuel pellets from the fuel pin cladding with material provided in a form which will lengthen the pin life.
2. Description of the Prior Art
The accepted material for the casing of fuel pins of a light water reactor is a zirconium alloy. The fuel pellets are stacked within a tube of this alloy. In dimension, these tubes are only in the order 7/16 inches in diameter, but their length is in the order of eleven feet. Water within the reactor is flowed over the pin surface while fissioning of the fuel pellets within the pin generates the heat absorbed by the water. The temperature of the water is raised to the order of 600.degree. F. and is then used to generate steam.
If rupture of the pin wall occurs, various fission products are released from inside the pin and contaminate the primary water system of the reactor. There may be one or more reasons for failure of the cladding, or tube wall.
Mechanical reasons for failure of the pin wall can be physical interactions between the pellets of ceramic fuel, such as UO.sub.2 or (UPu)O.sub.2, and the pin cladding. Thermal expansion rates of the pellets and their clad are different. The fuel pellets may also densify by approximately 1-2%. Thermal gradients within the pellets cause them to crack. These factors cause movement of the pellets relative to each other and the internal wall of the clad.
The result of the relative movement between pellets and clad is that some pellets restack, leaving small gaps along the length of the pellet column. The large hydrostatic pressure external to the pin causes the tube wall to creep down into any void space. The high stresses which result at the gap edges cause clad failure.
Thermally, there can be localized melting of the clad from within. If the fuel pellets, or material surrounding the fuel pellets, are fabricated from a mixture of powdered uranium oxide and plutonium oxide, agglomerates of plutonium oxide may form, ranging up to 500 microns in size. On a power transient, these agglomerates can melt. If an agglomerate of plutonium oxide comes in contact with a clad of zirconium alloy, there can be a localized meltdown which will rupture the clad.
Also, certain fission fragments produced in the fuel during reactor operation such as iodine and cesium can migrate from the hot fuel to the cooler clad. The iodine then reacts with the clad, causing it to fail by the mechanism of iodine stress corrosion cracking.
The prior art has at least one system which discloses insulation of fuel pellets from their clad. The disclosure of U.S. Pat. No. 3,778,348 specifically discloses microspheres of fuel in the annulus between the fuel pin clad and the pellets. Interaction between fuel pellets and clad is reduced by this isolation. However, the disclosed fuel microspheres are quite dense and hard. Material of this density and hardness severely reduces the space allowance of the annulus for dimensional changes in the fuel pellets. Also, the microspheres of fuel are disclosed as produced from a powdered form that is not purely homogeneous in the distribution of the plutonium. Therefore, agglomerates of plutonium oxide microspheres occur near the inner wall of the clad and generate local hot spots in the clad during core operation.
In summation, the prior art is seen to have both mechanical and thermal problems in operating fuel pins including ceramic fuel pellets isolated from the internal zircoloy wall of the pin with microspheres of fuel containing fissionable plutonium isotopes. First, the prior art material of hard, dense microspheres in the annulus between the pellets and wall does not provide sufficient volume to accommodate dimensional expansion of the pellets during core operation. The result is mechanical stress on the clad which can cause it to rupture.
Second, the prior art uses microspheres of material containing fissionable plutonium isotopes. The plutonium isotopes have not been evenly distributed and agglomerates of this element have been positioned next to the pin wall. The result can be localized meltdown of the zircoloy clad.